The invention relates to a bobbin winding device for generating a bobbin by winding a thread or bandlet onto a bobbin core.
Bobbin winding devices serve for winding threads or bandlets onto a bobbin core, which usually has a cylindrical or conical shape, so as to form a bobbin. In case of a known bobbin winding device as illustrated in side view in the schematic diagram of FIG. 1, a thread 1 gets to a first deflection pulley 2 of the bobbin winding device immediately upon its production. From there, the thread 1 runs on to a so-called “dancer roll” 3, which is a spring-biased, deflectable deflection pulley, and at the dancer roll it is deflected and tightened. From the dancer roll 3, the thread 1 runs on to another deflection pulley 4, and from there to a control device 5. The control device comprises thread deflection means 6, which may be configured as deflection bows, as well as a press roll 7, which at first, at the beginning of a bobbin winding process, presses the thread 1 against the peripheral surface of a bobbin core 8 and then, with a bobbin 9 building up from the supplied thread, against the periphery of the bobbin 9 that is building up. The bobbin core 8 is rotatable around an axis of rotation A. A thread guide 10 reciprocating the thread axially across the bobbin, thus providing for a regular bobbin structure in accordance with a predetermined winding pattern, is located on the control device 5 between the deflection means 6 and the press roll 7. So as to maintain a uniform pressure force of the press roll 7 against the bobbin 9 as the bobbin diameter D is increasing, the control device 5 is pivotable about a swivel axis C and is thus able to compensate for the increasing bobbin diameter. The arrow ρ(D) represents the deflection angle of the control device 5 depending on the bobbin diameter D.
The bobbin 9 or the bobbin core 8 is driven by a motor (not illustrated) at an angular velocity Ω. The tension in the thread 1 as it is being wound onto the bobbin 9 is critical for the quality of the bobbin winding. If the tension in the thread slackens, the motor speed must be increased in order to restore the desired tension. The dancer roll 3 serves for regulating the motor speed, which dancer roll already by itself provides for a certain compensation of the thread tension due to its spring bias. An increase in the motor speed is caused if the dancer roll 3 sags because of decreasing tension in the thread 1. If the dancer roll 3 rises because of an increase in the thread tension, the motor speed is reduced. Variations in the thread tension which necessitate changes in the motor speed will occur if the bobbin diameter D increases or if the thread production, and hence the supply of the thread to the bobbin winding device, accelerates or decelerates.
Another reason for variations in the thread tension is the axial movement of the thread guide 10, such as explained by way of the perspective illustration of FIG. 2. FIG. 2 shows the path of the thread 1 from the deflection pulley 4, via a deflection means 6 shaped like a straight deflection bow, through the thread guide 10 and, via the press roll 7, onto the bobbin 9. If the thread guide 10 is at the axial ends of the bobbin 9 during its axial reciprocation, the thread 1 is guided to the bobbin edge, thereby defining a path from the deflection pulley 4 to the bobbin edge which is longer than with the thread guide 10 situated at the center of the bobbin and with the thread 1 defining the path from the deflection pulley 4 to the bobbin center (illustrated by a broken line). Due to the shortened thread path, the thread becomes loose at the center of the bobbin. As the axial movement of the thread is generally performed at a relatively high frequency, the resulting thread-tension variation cannot be compensated for by regulating the rotational speed of the bobbin drive motor, since any kind of control unit, such as a PID controller, would either be too slow or would, under such conditions, be prone to unstable oscillation, i.e. an unstable control behavior. Therefore, so far it has been possible to contain the influence of the thread paths which have different lengths at the bobbin edge and at the bobbin center, respectively, on the thread tension merely by means of an as large as possible distance between the deflection pulley 4 and the press roll 7. If the distance is larger, the angle extending between the deflection pulley 4 and the two positions of the thread 1 at the bobbin edges and hence also the factor (cosine) of longitudinal deformation will become smaller.
Again with reference to the illustration of FIG. 1, it is evident that the thread-path length x(ρ) between the stationary deflection pulley 4 and the deflection means 6 attached to the control device 5 changes depending on the bobbin diameter D, since an increase in the bobbin diameter will result in a deflection of the control device 5 in the direction of the deflection pulley 4. With a deflection of the control device 5, the distance z(ρ) between the press roll 7 located on the control device 5 and the stationary deflection pulley 4 will change as well. The distance y between the press roll 7 and the deflection means 6 remains constant irrespective of the deflection of the control device 5.
The effects of incorrect thread tensions on the quality of the bobbin are enormous. The choice of thread tension for the winding process will not be discussed in full detail now, however, in general terms it can be said that an incorrect thread tension and in particular a varying tension of the thread between the bobbin edge and the bobbin center will result in a thread that falls off the edge of the bobbin, such as illustrated in FIG. 3. In FIG. 3 it can be seen that the thread 1 has fallen off the edge of the bobbin 9 and onto the bobbin core 8 and would subsequently wind itself around the bobbin core. This falling off of the thread might have an impact on the production capacity already during the process of manufacturing the bobbin and could lead to dead halts, or might have an impact when using the bobbin later on, for example when weaving the thread, and could then lead to dead halts or breakdowns.
The fact that the thread does not fall off is thus one of the most important features of a bobbin. With the known bobbin winding devices it was, however, difficult to fulfill that criterion in a satisfactory manner. Particularly as a result of the high winding frequency, it was not possible to compensate for the varying thread tensions between the bobbin edge and the bobbin center by the use of motor control systems.